An ultrasonic detector array having a plurality of ultrasonic oscillators which are linearly arranged is used to measure a diameter of a blood vessel (such as an arterial vessel) located below a skin of a live body. Patent Document 1 discloses an example of an ultrasonic blood vessel inspecting apparatus configured to measure a lumen diameter and a thickness of an intima-media complex of the above-described blood vessel, by using an ultrasonic probe of an H-type which consists of mutually parallel first and second ultrasonic detector arrays and a third ultrasonic detector array connecting intermediate portions of the first and second ultrasonic detector arrays, and which is positioned such that the third ultrasonic detector array is parallel to the blood vessel and aligned with a centerline of the blood vessel. The ultrasonic blood vessel inspecting apparatus disclosed in this Patent Document 1 is provided with a multi-axes drive device which is operated to position the above-described ultrasonic probe on the basis of a first short-axis ultrasonic image obtained by the above-described first ultrasonic detector array and a second short-axis ultrasonic image obtained by the above-described second ultrasonic detector array, such that a distance between the above-described first ultrasonic detector array and the centerline of the above-described blood vessel and a distance between the above-described second ultrasonic detector array and the centerline of the blood vessel are equal to each other. Further, the above-described multi-axes drive device is operated to position the above-described ultrasonic probe such that an image of the above-described blood vessel is positioned in a widthwise central portion of each of the above-described first and second short-axis ultrasonic images.